Audiologist frequently find that patients may benefit by wearing two hearing aids. If there is adequate residual hearing in both ears, the result is generally an improved ability to localize, or determine the location of, a source of sound. With good localization ability, patients are also able to hear a selected source in the presence of ambient noise (i.e., the "cocktail party effect"). As a result, prescription and purchase of two hearing aids is now common. However, no instrumentation is available to audiologists that would enable them to assess the ability of the client to localize sound sources either in aided or unaided conditions. As a result, localization ability is not considered in the fitting of hearing aids or in the assessment of auditory function. Such procedures are essential to ensure that the patient obtains the maximum possible benefit from binaural aids in light of the considerable additional cost and inconvenience of wearing and maintaining two hearing aids. The work on this grant project is aimed at the development of a compact device that will allow accurate measurement of localization ability to be made economically and quickly. The theory of the method to be used has been evaluated experimentally during Phase I. In Phase II, a practical device is to be designed, thoroughly tested, and refined in clinical use.